Mechanisms of speciation are not well understood despite
decades of study. Recent work has focused on how natural and sexual selection
cause sexual isolation. Here, we investigate the roles of divergent natural and
sexual selection in the evolution of sexual isolation between sympatric species
of threespine sticklebacks. We test the importance of
morphological and behavioral traits in conferring sexual isolation and ask to
what extent these traits have diverged in parallel between multiple, independently_evolved species pairs. We use the patterns of
evolution in ecological and mating traits to infer the likely nature of
selection on sexual isolation. Strong parallel evolution implicates ecologically_based divergent natural and/or sexual
selection, whereas arbitrary directionality implicates non-ecological sexual
selection or drift. In multiple pairs we find that sexual isolation arises in
the same way: assortative mating on body size and
asymmetric isolation due to male nuptial color. Body size and color have
diverged in a strongly parallel manner similar to ecological traits. The data
implicate ecologically_based divergent natural and
sexual selection as engines of speciation in this group.